One outstanding example of this is SN 2010lp. While it appeared to be similar
spectroscopically to SN 1991bg near maximum, showing a cool spectrum and low
line velocities [1]}, it
was not very sub-luminous. An estimated optical peak luminosity of \(\sim 2.5\times 10^{43}\)  erg s\(^{-1}\)  [2]} is comparable to that of
low-luminosity, transitional SNe Ia such as 1986G [3]}. The
decline rate of the light curve after maximum,
[2]}, however, was similar to that of normal SNe Ia. In the
nebular phase, SN 2010lp was again similar spectroscopically to SN 1991bg,
except that, instead of showing narrow [Fe iii] lines at low velocities, it
displayed [O i] 6300,6363 Å emission [5]}. Not only had these
lines not been seen before in any SN Ia, but they were very narrow in
SN 2010lp, suggesting that they originate in the centre of the ejecta, and they
showed two components, one blue-shifted and the other red-shifted by similar
velocities. Double-peaked profiles had been observed only in the Na i D line in
SN 2007on [6]}, but the two components were not as distinct
[7]}. The clear separation of the two components of the [O i]
line in SN 2010lp suggests that the emission originates in two separate,
low-velocity blobs of material, as both spherically symmetric options, a shell
and a disc configuration, would not create such sharp separation between the two
peaks. A transition to an [Fe iii]-dominated phase was not observed in
SN 2010lp, indicating that at low velocities iron was absent, or if it was
present either the density was not very low or that the presence of oxygen led
to a higher electron density \(n_e\) , making recombination easier. The late-time
spectrum of SN 2010lp was taken at an epoch of 264 days after maximum.
[5]} compare it to the spectrum of SN 1991bg obtained
\(\sim 200\)  days after peak, showing the impressive difference between them
despite the underlying similarity.
